M. Adam Davis wrote:
> Your particular message talks about two easy items to dismiss - small
> private aircraft generally fly low and slow, and so are less affected
> by items in this discussion.  They are still relevant, but the
> extremes aren't so bad.  Grounded planes are not special and have
> little to do with this topic except as pertains to how cell phones
> might possibly affect the airplane instruments.
> 
> There are several factors in operation here.

Nice summary Adam.  I have only a small nit-pick, that might be 
interesting to some reading along.

In CDMA phones, the control of tower switching is handled by the 
back-end equipment, and not the phone.  The phone is "told" what to do 
by the tower it's communicating with.

The tower bases these instructions on a number of algorythms, including 
received signal strength (RSSI), bit-error rate (BER) and other things 
like pre-programmed knowledge of the terrain/location a particular base 
station is located in.

One of the factors the tower can control in the phone itself is the 
phone's transmitter power level.

This becomes important later.

The only time the phone transmits at full power and has complete control 
of its own transmitter is during an initial power-up "yell" to check in 
with someone.  Even then, it usually is going to listen first for towers 
it can hear that are presenting a common data channel carrier signal and 
might have the intelligence to "match power levels" by transmitting a 
weaker signal if it determines that the RSSI of the tower is strong 
enough that it probably doesn't need to blast a full-power "hello?" out...

I've had discussions with people who design the base-station side and 
learned a lot, but the phone side is manufacturer-specific unless 
they're going for a certification that they interoperate "correctly" to 
a certain level.  Most do, but these certification procedures are 
between them and the base-station manufacturers (and in the case of CDMA 
- QualComm, the licensors of the technology they created...), and are 
not available to the general public since they're released under NDA 
only, along with the license to use the technology.

> The reason the cell phone repeater helps with this problem is that it
> allows all the cell phones to find a clear "tower" so they aren't
> generating a lot of traffic trying to find a better tower, and since
> it's inside the skin they can transmit at their lowest power.  The
> repeater would have to deal with doppler effects from the ground
> tower(s) it relays from, but that's easily resolved by proper choice
> of tower.

The cell phone repeaters being designed for aircraft in-flight cell 
phone use are usually mini-cell-sites from the phone's perspective.

The "airborne base station" (in CDMA systems, especially CDMA 2000 and 
above) has complete control over the phone's transmitted power level.

As you pointed out, no one wants to find out the hard way about how a 
high power transmitter (or group of them) on-board interact with older 
avionics, etc.

In the "cell phone repeater" type system, if the on-board repeater is up 
and active, the system will tell the phone not to transmit above a 
particular power level, and will easily be within range of even the 
lowest transmitter power level of most consumer-grade cell phone 
transmitters.

Then -- the device will couple the audio over to another 
flight-certified and designed RF system to carry that signal to the 
ground in a safe and sane way.

Nate
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